Infection and invasion by influenza viruses requires the intermediacy of sialic acid residues on the surface of the host cell. Sialic acid and neuraminic acid are used interchangeably. Similarly, sialidase and neuraminidase are used interchangeably. Initial attachment of the virus to the host cell occurs via the binding of the virus to these sialic acids (charged, 9-carbon sugars) through the hemagglutinin protein of the virus. Once inside the cell the virus replicates by taking advantage of the host cellular machinery. However, in order to remain optimally infective, the virus has evolved a neuraminidase that cuts off the sialic acid from the host cell surface to assist the virus in escaping the host cell to infect other cells. Failure to cut off the sialic acid from the host cell surface, results in retention of virus through attachment to the host cell.
The GH33 family of neuraminidases contains all the sialidases except the viral enzymes (GH34 family). The GH33 and GH34 families are distinct structurally and by sequence (See Cantarel B L. et al. (2009); and Henrissat B. and Davies G J (1997) for background on Family classifications). Previous work has demonstrated that 2,3-difluorosialic acids are effective inhibitors of GH33 neuraminidases and that GH33 neuraminidases proceed through a covalent intermediate (see for example, Watts, A. et al. (2003); Amaya, M. F. et al. (2004); Watts, A. G. and Withers, S. G. (2004); Watts, A. G. et al. (2006); Newstead, S. et al. (2008); Damager, I. et al. (2008); and Buchini, S. et al. (2008)).
The most probable mechanism for the GH34 sialidase (i.e. viral sialidases) reported in the literature is one involving an ion-pair intermediate (von Itzstein M. (2007)).
A number of compounds are known to inhibit neuraminidases. Some well known neuraminidase inhibitors are alkene containing sialic acid analogues (for example: Laninamivir CAS #203120-17-6; Oseltamivir (Tamiflu) CAS #204255-11-8; and Zanamivir (Relenza) CAS #139110-80-8; see also U.S. Pat. No. 5,360,817; and Ikeda et al. Bioorganic & Medicinal Chemistry (2006) 14:7893-7897). Fluorinated sugar derivatives with (reactive) fluoride leaving groups have been shown to be inhibitors of a range of “retaining” glycosidases and function via formation of particularly stable glycosyl-enzyme intermediates (for example, Hagiwara et al. (1994); and Buchini et al. (2008)). These reagents are quite specific with respect to their target enzymes, have been shown to be highly bio-available, and even capable of crossing the blood-brain barrier. Such inhibitors are mechanism-based in their action, making the development of resistance by viruses difficult, whereby any mutations in the viral enzyme that reduce the inhibition must necessarily reduce the efficiency of the enzyme on the natural substrate, sialic acid and therefore less likely to be tolerated.